Armored electrical resistors are generally used in household appliances which are in contact with water, such as washing machines, dishwashers, boilers or the like. Typically, these resistors are formed by a resistance wire coaxially inserted in a tubular metal casing, which is filled with electrically insulating powder, e.g. magnesium oxide, which is then consolidated. The resistance wire is connected with pins which protrude from the ends of the casing and which have the function of connecting to the electrical power supply system.
Closing elements which are sealed to the casing are present at each end of the casing.
Disadvantageously, the sealing is generally performed by means of resins, e.g. epoxy or polyurethane. This implies the need to implement particularly complex polymerization processes by means of specific apparatuses, requiring a high degree of accuracy.
Furthermore, the armored resistors are disadvantageously formed by a relatively high number of parts, of which also several parts forming the casing, which are mutually joined, in general by brazing.
The armored resistors may envisage safety devices, typically a thermal fuse, which has the function of interrupting the electrical current in case of unexpected overheating of the heating element. In particular, the thermal fuse is usually arranged outside the tubular casing and electrically connected to the resistance wire by means of brackets, thus making the entire armored resistor bulkier and of more complex construction.
Disadvantageously, an armored resistor of this type has some drawbacks, in particular due to the length of the armored resistor itself. For example, for some applications it is desirable to have an armored resistor which does not exceed a given length. One of the reasons for reducing the size of the armored resistor is that the space available for housing it, is rather small. Another reason is related to excessive resistance, due to the excessive total length of its components, which implies a slower heat transmission from the heating element to the thermal fuse and a lower operating sensitivity of the latter, due to the longer distance between fuse and heating element, with consequent larger dimensions and longer reaction times of the thermal fuse. Less reliability and a shorter working life thus derive.
Although an armored resistor in which the thermal fuse is housed in a closing element has been suggested, such a solution is not free from drawbacks. A disadvantage is that the casing must be pressed in order to provide a better heat exchange between resistance wire and its casing. Such an operation is performed by making the entire armored resistor pass through a roller mill. This implies that the part in which the thermal fuse is housed is also subjected to the rolling pressure with the consequent risk of damaging the thermal fuse, which is a particularly delicate component. Disadvantages also appear when a thermal fuse is not envisaged; indeed, a roller mill cannot be used for compacting armored resistors shorter than a given length.
It is thus felt the need to make an armored resistor which allows to overcome the aforesaid drawbacks.